1. Field of the Invention
This invention relates in general to a repeater transmission protocol and data buffering scheme, and more particularly to a centrally arbitrated repeater device for Local Area Networks (LANs) having increase bandwidth utilization and minimum buffer memory requirements.
2. Description of Related Art
A local area network (LAN) is a communication system that provides interconnection among a number of independent computing stations within a small area. Data is usually transmitted using a tree or bus typology. Data is transmitted in a network and units are usually referred to as "frames" or "packets". Each packet includes control information such as the address of the packet source (transmit station) and the address of the packet destination (receive station). These concepts have been expanded for metropolitan area networks (MANs) and Wide Area Networks (WANs).
Because a wide variety of physical, electrical and procedural characteristics available to designers of equipment for local area network, it has become widely acknowledged that certain standards must be observed. For example, the International Organization For Standardization (ISO) has developed a voluntary Open Systems Interconnection (OSI) model which defines a general computer system architecture. An "open" system may be implemented in any way provided that it confirms to a minimal set of OSI standards that allows it to communicate with other "open" systems. A number of local area network protocol standards have been developed by the institute of electrical and electronics engineers (IEEE) 802 committee. One of these standards, the IEEE 802.3 standard, defines a protocol for a bus/tree local area network. As discussed below, the IEEE 802.3 standard defines a bus/tree protocol that implements the carrier sense multiple access with a collision detection (CSMA/CD). The standard also defines a media access control (MAC) function for transmitting packets to and receiving packets from the transmission media as well as packet structure and the interaction that takes place between MAC entities in the network.
The length of a bus/tree network can be extended by connecting together a number of medium "segments" using "repeaters". A repeater comprises two or more MAUs and associated logic joined together and connected to two or more different segments of the network medium by corresponding AUIs. The repeater passes retimed digital signals in both directions between the two segments, amplifying and regenerating the signals as they pass through.
Repeaters in Local Area Networks (LANs) are simple devices for broadcasting data packets originating at one port of the repeater to all other ports. Multiple end stations, i.e., Data Terminal Equipment (DTEs), connect to repeaters via coaxial cables, or twisted-pair wiring, or optical fiber in the LAN.
The main task of a repeater is to handle "collisions." Collisions happen when multiple ports attempt to transmit data at roughly the same time, such that there is a conflict in the network as to which port "wins."
Because all stations in a network share a common transmission medium, only one station may transmit at a time. A transmitted packet propagates through the medium, is received by all stations on the network and is copied by the destination station to which it is addressed.
Current design of repeaters are based on either IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) standard for Local Area Networks, IEEE 802.12 Demand Priority Access Method (DPAM) standard for Local Area Networks, or RTS/CTS scheme used in RS-232.
According to the CSMA/CD technique, if a repeater wishing to transmit senses that the medium is idle, then it transmits. If the repeater senses that the medium is busy, it continues to listen to the medium until it senses that the medium is idle and then immediately transmits.
Carrier Sensing Multiple Access with Collision Detection (CSMA/CD) repeaters replace all colliding data packets with a "JAM" signal such that every port detects the collision. Then each port "backs off" for a random amount of time and tries to resend the data packet.
In the Demand Priority Access Method (DPAM), the repeater acts as a central arbitrator. It "grants" access to the LAN to each port and if the DTE at the port has any data to send, it "captures" the LAN.
In the Request-to-Send (RTS), Clear-to-Send (CTS) scheme, such as that used in RS-232, every DTE that has data to send, first issues an RTS to the repeater. The repeater then handles all RTSs on a first-come-first-served basis and issues a CTS at the selected port. The selected DTE then transmits the data.
The difficulty with all of the above schemes is that the bandwidth utilization of the LAN declines drastically as LAN speeds increase. This is because of the round trip delay associated with the wiring that connects the DTE to the repeater. For instance, at LAN speeds of 1 Gb/s, the 1.2 microsecond round-trip delay of 100 m twisted-pair cooper wiring (which is ubiquitous in current installations) translates to an equivalent of 1.2 Kbits of data. If we were to use an RTS/CTS scheme, the minimum packet size were 512 bits, and all ports are sending minimum size packets, the bandwidth utilization of the repeater is less than 30%. This is because for more than two-thirds of the time, the repeater is sending CTS signals and waiting for a response instead of doing data transmission.
It can be seen that there is a need for a repeater scheme that minimizes the amount of buffering necessary at the repeater.
It can also be seen that there is a need for increase bandwidth utilization.